The present invention relates to a ring section furnace device for the calcination of carbon bodies including a number of serially connected chambers, each containing several cassettes, the walls of which are fitted with vertical and/or horizontal flue gas channels and with a gas exhaust system through a ring duct.
For the production of carbon bodies for furnaces for aluminum electrolysis or electro-metallurgical processes, special furnaces are used for thermal treatment (baking or calcination) of the carbon bodies.
The carbon bodies are produced in the required shape from a mixture of crushed coke or anthracite and a binding agent containing, for example, coal tar and pitch.
At room temperature this mixture of coke and binding agent is stiff, but it softens at temperatures above 120.degree. C. and release's low-volatile components from the binding agent. At prolonged heating to a maximum of 1300.degree. C., the mass hardens and changes its physical properties such as electrical conductivity and resistance to oxidation.
Uncalcinated carbon bodies are often called "green carbon". Such green carbon can be of a considerable weight of several tonnes and of lengths of two meters or more. Special measures must be implemented in order to prevent deformation of the coal when it passes through a temperature range at which the coal is in its soft state.
The green carbon is placed in the furnace in deep shafts known as pits or cassettes, which are defined by walls built of fireproof brick. The gap between the coal and the cassette walls is filled with coke to support the coal. The coke gravel also serves to protect the coal from burning.
A number of cassettes are connected to each other in a so-called chamber. The walls between the cassettes are fitted with flue gas channels. Heat is transferred to the coal by the gases being passed through these channels.
Gases from one chamber are led via channels to the adjoining chambers. In this way the gases can be led through a number of serially connected chambers in a so-called combustion zone. The most frequently used fuels are oil or gas.
Flue gas discharge and burner equipment are moved from chamber to chamber.
A large furnace is often fitted with two rows with the chambers connected to each other as parallel rows. At the end of a chamber row the gas flows are connected with channels to the parallel chamber row. In this way the chambers form a ring. For this reason this type of furnace for baking carbon bodies is known as a ring section furnace.
A ring section furnace may contain several combustion zones in which the temperature is adjusted in accordance with a set program. The first chambers in a combustion zone have low temperatures. After these follow chambers with higher temperatures, chambers for heat recovery and, as the final link in the combustion zone, chambers in which the coal is cooled. Ring furnaces for thermal treatment of carbon bodies can be divided into two main categories, closed and open furnaces.
In a conventional closed design the space above each chamber is covered with lids. Such lids are removed when the chambers are to be cooled with a subsequent insertion of green carbon after the calcinated carbon bodies have been removed.
Due to the special properties of the carbon bodies, it is necessary to avoid excessive temperature gradients during calcination which would result in cracks in the finished product. Each chamber must consequently be operated in accordance with an exact time and temperature program.
Heat supply usually takes place in the first part of the zone, i.e. up to 600.degree. C., by using the gas heat from the last part of the combustion zone. At a later stage in the temperature interval, from 600.degree. C. to the required maximum temperature (1200.degree.-1300.degree. C.), it becomes necessary to add heat through the above-mentioned combustion of gas or oil.
In the cooling area the cassette walls are cooled with air until the carbon bodies can be removed without any risk of oxidation. The furnace has been designed for maximum utilization of the heat which is absorbed by the cooling air by passing the surrounding air through 1-3 chambers during cooling and onwards into the combustion zone where it is used as combustion air.
The combustion zone is moved by moving the oil or gas burners from one chamber to the next. The frequency of this relocation is known as the heating progress and determines the combustion zone capacity.
As mentioned, it must also be possible for each chamber to be connected to an exhaust system when the chamber is to be connected to the combustion zone. This connection is generally established by fitting an exhaust pipe or manifold, possibly with a fan, between the chamber in question and a joining pipe at the exhaust duct surrounding the furnace. This exhaust duct is known as the ring duct and is kept under ventilating pressure by a main fan.
In the invention in question, for closed furnaces connection to the chamber takes place on the chamber lid itself. For open furnaces recovery devices in the form of manifolds are connected to openings in the part walls between the chambers.
In closed ring section furnaces several cassettes are built together in one chamber under a joint lid. In relation to the flue gases and the material which is to be calcinated, the cassettes in a chamber are connected in parallel, whereas the chambers are serially connected. There are horizontal flue gas channels in the room or space below the chamber, whereas there is free gas flow in the room or space below the chamber lid above the cassettes. The gas channels in the cassette walls connect the room below the chamber lid and the rooms below the chamber. In closed ring section furnaces the flue may be supplied either in separate vertical furnace shafts or preferably by the flue being added fully or in part to the room above or below the cassettes as shown in Norwegian Patent No. 150,029 (corresponding to U.S. Pat. No. 4,552,530).
In closed furnaces without furnace shafts the channels in each cassette wall are divided into two by a dividing wall in the room below the cassettes. The flue gases are consequently led up through one half of the wall and down through the other half of the wall. In open furnaces the chambers are serially connected with parallel connection of flue gas flow above or below the individual chamber.
Before the gases reach the main fan, they normally pass through a purifying plant in which soot, tar fumes and other impurities are removed.
In order to increase the speed of cooling of for the carbon bodies in the cassettes, individual cooling fans are used which either force or suck out the surrounding cooling air through the flue gas channels.
This cooling air cannot be led into the combustion zone in its entirety as it would disturb the pressure conditions and gas quantity balance in the system. It is consequently let out into the surrounding factory.
In this connection the chamber lids in the familiar design with closed furnaces are removed from the chambers to which cooling fans are attached. In open furnaces manifolds which force or suck cooling air through the cassette wall are fitted on the cassette walls and/or on the part walls between the chambers. This can only be done when the temperature in the cassettes has fallen below a certain level.
The cooling air which is let out in this way contains impurities such as SO.sub.2, soot and ash components from the coke used. These impurities contribute to a deterioration of the working environment and increase polluting emissions into the environment.